1. Field of the Invention
The present invention relates generally to channel estimation in an Orthogonal Frequency Division Multiplexing (OFDM) receiving apparatus, and more particularly, to providing a robust synchronization position adjustment algorithm to improve channel estimation in an OFDM receiving apparatus.
2. Description of the Related Art
FIG. 1 is a diagram illustrating an OFDM system, which includes an OFDM transmitting apparatus 102 and an OFDM receiving apparatus 110. The OFDM transmitting apparatus 102 receives data for modulation in a modulation unit 104. The modulated data is provided to an Inverse Fast Fourier Transform (IFFT) unit 106 and then a digital to analog conversion unit 108. A baseband OFDM signal is then transmitted from the OFDM transmitting apparatus 104. The baseband OFDM signal is received at an analog to digital conversion unit 112 of the OFDM receiving apparatus 110 and then provided to a Fast Fourier Transform (FFT) unit 114 before demodulation in a demodulation unit 116. Data is then output from the demodulation unit 116 of the OFDM receiving apparatus 110.
Accurate channel estimation is a crucial factor in the performance of a Physical (PHY) layer in an OFDM system. Channel estimation is typically performed between FFT and demodulation in the OFDM receiving apparatus. A channel may be estimated with training field preambles, such as, for example, Legacy Long Training Field (L_LTF), High Throughput Long Training Field (HT_LTF), and Very High Throughput Long Training Field (VHT_LTF) in Institute of Electrical and Electronics Engineers (IEEE) 802.11a/g, IEEE 802.11n, and IEEE 802.11ac, respectively. Zero forcing is an algorithm that is commonly adopted to estimate channel response along all used subcarriers. Zero forcing channel estimation does not exploit coherence among neighboring subcarriers, which increases estimation errors, especially when white Gaussian noise is presented.
In Jan-Jaap Van De Beek et al., “On Channel Estimation in OFDM System,” Proceedings of the IEEE Vehicular Technology Conference, July 1995, pp. 815-819, a Minimum Mean Square Error (MMSE) algorithm is proposed to obtain optimal linear estimation by exploiting the correlation between subcarriers. However, this solution is impractical to implement in hardware due to its high complexity.
Attempts to obtain suboptimal solutions have also been made, such as in Darryn Lowe and Xiaojing Huang, “Adaptive Low-Complexity MMSE Channel Estimation for OFDM,” 2006 International Symposium on Communications and Information Technologies (ISCIT), Bangkok, Thailand, 18-20 Oct. 2006, and Hsuan Yu Liu et al., “Combining Adaptive Smoothing and Decision-Directed Channel Estimation Schemes for OFDM WLAN Systems,” Proceedings of the 2003 International Symposium on Circuits and Systems, pp. 149-152. However, these proposals assume an ideal synchronization position for a packet, which is very difficult to achieve, especially in fading channel or multiple transmitter scenarios due to the well-known pseudo-multipath problem, even though some attempts have been made to improve it. See, e.g., U.S. patent application Ser. No. 12/617,537, filed Nov. 12, 2009.
Although current synchronization in an OFDM wireless Local Area Network (LAN) cannot accurately detect the exact start of each symbol of an OFDM packet, this may not cause a serious problem due to the existence of a guard interval. A packet offset is introduced to start FFT data sampling within the guard interval to mitigate the effects of an inaccurate sync position. However, when there is an error in synchronization, the channel will no longer be flat for an Additive White Gaussian Noise (AWGN) channel, as both the real and imaginary parts will display sinusoidal oscillation. Applying a smoothing filter, which is not optimal for such synchronization error, to such an oscillating channel can introduce distortion. The above-described phenomenon becomes even more serious for a multiple transmitter antennas scenario due to synchronization uncertainty from the pseudo-multipath phenomenon caused by cyclic shift.